The Problem
Known adaptive equalizers with fractional tap spacing--reference: (1) G. Ungerboeck, "Fractional Tap-Spacing Equalizers And Consequences For Clock Recovery In Data Modems", IEEE Trans. On Communications, Vol. Com. 24 No. 8, August, 1976, Pages 856-864; (2) U.S. Pat. No. 4,343,759--employ a training process preceding real data transmission to establish an optimized ensemble of tap coefficients, with optimum tap positioning. After training, tap positions of such coefficients are held fixed while non-training data symbols are received. In this mode, coefficient magnitudes are adjusted slightly after each symbol is received in order to minimize the mean squared error in equalizer output relative to an ideal symbol level. Sample timing is also adjusted during post-training reception in order to maintain synchronism between the sampling process and the transmission source, and thereby minimize errors in reception processing.
Adjustable sample timing requires use of variable oscillators which are relatively costly, particularly for high baud rate transmissions. Furthermore, there are communication situations in which adjustable sample timing would be very difficult to accomplish (refer to the above cross-referenced copending application by Davis et al). A more efficacious scheme is presented presently.